Texinfo - The GNU Documentation System

Texinfo is the official documentation format of the GNU project. It was invented by Richard
Stallman and Bob Chassell many years ago, loosely based on Brian Reid's
Scribe and other formatting languages of the time. It is used by many
non-GNU projects as well.

Texinfo uses a single source file to produce output in a number of
formats, both online and printed (dvi, html, info, pdf, xml, etc.). This
means that instead of writing different documents for online information
and another for a printed manual, you need write only one document. And
when the work is revised, you need revise only that one document. The
Texinfo system is well-integrated with GNU
Emacs.

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